1. Field of the Invention
The present invention relates to a novel ferroelectric chiral smectic liquid crystal mixture and a liquid crystal element comprising the same. In particular, the present invention relates to a novel liquid crystal mixture having an improved contrast ratio and memory capability, and a liquid crystal element comprising the same which can be used in an optical shutter or a display device.
2. Description of the Related Art
With the recent progress of the information society, importance of a display device as one of man-machine interfaces has been greatly increased. Among such display devices, a flat panel display device such as a liquid crystal display (LCD) is quickly spread since it has various advantages such as a thin thickness, a light weight, a low driving voltage, a low power demand, and the like. Among the liquid crystal devices represented by the liquid crystal display, a matrix-addressed liquid crystal device having a large capacity of information includes two types of driving systems, namely an active-matrix-addressed system and a passive-matrix-addressed system.
In the active-matrix-addressed system, a thin film transistor or diode made of polysilicon or amorphous silicon is connected to each picture element as a non-linear element. However, the active-matrix-addressed system may have some problems in assembling a large area display, lowering a production cost or increasing a density because of complicated production steps and low yield. In view of the cost and productivity, the passive-matrix-addressed system is advantageous.
As liquid crystal devices of the passive-matrix-addressed LC display which are currently practically used, TN liquid crystal and STN liquid crystal devices are mainly used. However, their optical response utilizes an average orientation of a molecule-axis of the liquid crystal in a specific direction based on anisotropy of a dielectric constant of the liquid crystal molecule, which is caused by the application of an electric field. Then, a limit of an optical response speed of such devices is in the order of millisecond, and such response speed is insufficient in view of the increase of an amount of information. When the number of scanning lines is increased to increase the capacity of information, the decrease of the contrast ratio or cross-talk cannot be avoided inevitably. Those are the essential problems since the TN or STN liquid crystal device has no memory property (bistability). To solve such problems, various driving methods such as a dual frequency diving method, a voltage averaging method, a multimatrix method, and the like have been proposed. But, they cannot provide the fundamental solution of the problems. By such methods, it is difficult to increase the capacity or the density. Further, the TN or STN liquid crystal device has serious problems such as limitation of a view angle or quality of display.
To solve the essential problem of the above liquid crystal devices, in 1980, N. A. Clark and S. T. Lagerwall proposed a liquid, crystal element utilizing a liquid crystal having bistability (see U.S. Pat. No. 4,367,924 and Japanese Patent KOKAI Publication No. 107216/1981). As the liquid crystal having the bistability, there is used a ferroelectric liquid crystal which has a chiral smectic C phase.
One of the advantages achieved by the use of the ferroelectric liquid crystal is that it has the bistability. The bistability is a property such that, when a ferroelectric liquid crystal is held between a pair of glass plates each carrying a transparent electrode, the ferroelectric liquid crystal has two different optically stable states depending on directions of the applied electric field, and the two optically stable states are maintained after the removal of the electric fields. Because of such property, the liquid crystal device utilizing the ferroelectric liquid crystal is expected not to suffer from the decrease of the contrast ratio or cross-talk even when the number of scanning lines is increased.
Another characteristic of the ferroelectric liquid crystal resides in a high response speed. That is, the optical response of the ferroelectric liquid crystal is about 1000 times faster than that of the TN or STN liquid crystal, since the former utilizes a change of the orientation of the liquid crystal molecules caused by the direct interaction between spontaneous polarization of the ferroelectric liquid crystal and the electric field.
Accordingly, the ferroelectric liquid crystal has the following essential characteristics:
(1) It has two optically stable states, and those optically stable states are maintained after the removal of the electric field (bistability), and
(2) The above two optically stable states can be switched in a microsecond order (high response speed).
In addition, in the ferroelectric liquid crystal device, the liquid crystal molecules respond to the electric field in parallel with the substrate and a cell thickness is made thin, so that dependency of the display on the view angle is small (large view angle).
Consequently, the ferroelectric liquid crystal device does not require the expensive non-linear element as in the case of the active-matrix-addressed system, and is expected to provide a high quality large display which can achieve the large information capacity and high quality display by the passive-matrix-addressed system.
Then, a large number of studies on the liquid crystal materials having ferroelectricity have been reported. To use the ferroelectric liquid crystal device practically, there still remain some problems to be solved. In particular, it is highly desired to provide a liquid crystal which has good orientation and memory capability.